Method for producing semiconductor device

ABSTRACT

In producing a thin film transistor, after an amorphous silicon film is formed on a substrate, a nickel silicide layer is formed by spin coating with a solution (nickel acetate solution) containing nickel as the metal element which accelerates (promotes) the crystallization of silicon and by heat treating. The nickel silicide layer is selectively patterned to form island-like nickel silicide layer. The amorphous silicon film is patterned. A laser light is irradiated while moving the laser, so that crystal growth occurs from the region in which the nickel silicide layer is formed and a region equivalent to a single crystal (a monodomain region) is obtained.

This is a Divisional application of Ser. No. 08/525,167, filed Sep. 8,1995 now U.S. Pat. No. 5,712,191.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for producing a semiconductordevice using a crystalline thin film semiconductor.

2. Description of the Related Art

Recently, much attention is paid on a transistor constructed of a thinfilm semiconductor formed on a glass or quartz substrate. Such a thinfilm transistor (TFT) is constructed of a thin film semiconductor havinga thickness of several hundreds to several thousands of angstroms (A)formed on the surface of a glass substrate or a quartz substrate(insulated gate field effect transistor).

TFTs are used in an application field such as the field of an activematrix type liquid crystal display device. An active matrix type liquidcrystal display device has several hundred thousands of pixels arrangedin a matrix, and TFTs are provided to each of the pixels as switchingelements to realize a high quality image display. Practically availableTFTs designed for active matrix type liquid crystal display devicesutilize thin films of amorphous silicon.

However, TFTs based on thin films of amorphous silicon are stillinferior in performance. If a higher display function is required as aliquid crystal display of an active matrix type, the characteristics ofTFTs utilizing an amorphous silicon film are too low to satisfy therequired level.

Further, it is proposed to fabricate an integrated liquid crystaldisplay system on a single substrate by using TFTs to realize not onlythe pixel switching, but also the peripheral driver circuit. However, aTFT using an amorphous silicon thin film cannot constitute a peripheraldriver circuit because of its low operation speed. In particular, abasic problem is that a CMOS circuit is unavailable from a amorphoussilicon thin film. This is due to the difficulty in implementing apractical P-channel type TFT by using amorphous silicon thin film (i.e.,the TFT using an amorphous silicon thin film is practically unfeasibledue to its too low performance).

Another technology is proposed to integrate other integrated circuitsand the like for processing or recording image data, etc., on a singlesubstrate together with the pixel regions and the peripheral drivercircuits. However, a TFT using a thin film of amorphous silicon is tooinferior in characteristics to constitute an integrated circuit capableof processing image data.

On the other hand, there is a method for manufacturing a TFT using acrystalline silicon film which is far superior in characteristics ascompared with the one using a thin film of amorphous silicon. The methodfor manufacturing TFT comprises the steps of: forming an amorphoussilicon film; and modifying the resulting amorphous silicon film into acrystalline silicon film by subjecting the amorphous silicon film toheat treatment or to laser irradiation. The crystalline silicon filmobtained by crystallizing an amorphous silicon film generally yields apolycrystalline structure or a microcrystalline structure.

As compared with a TFT using an amorphous silicon film, a TFT having farsuperior characteristics can be obtained by using a crystalline siliconfilm. In mobility which is one of the indices for evaluating a TFT, aTFT using amorphous silicon film yields 0.5 to 1 cm² /Vs or lower (in anN-channel TFT), but a TFT using a crystalline silicon film has amobility of about 100 cm² /Vs or higher in an N-channel TFT, or about 50cm² /Vs or higher for a P-channel TFT.

The crystalline silicon film obtained by crystallizing an amorphoussilicon film has a polycrystalline structure. Hence, various problemsarise due to the presence of grain boundaries. For example, carrierswhich move through the grain boundaries greatly limit the withstandvoltage of the TFT. The change or degradation in characteristics whichoccurs in high speed operation is another problem. Further, the carrierswhich move through the grain boundaries increase the OFF current (leakcurrent) when the TFT is turned off.

In manufacturing a liquid crystal display device of an active matrixtype in a higher integrated constitution, it is desired to form not onlythe pixel region but also the peripheral circuits on a single glasssubstrate. In such a case, it is required that the TFTs provided in theperipheral circuit operate a large current to drive several hundredthousands of pixel transistors arranged in the matrix.

A TFT having a wide channel width must be employed to operate a largecurrent. However, even if the channel width should be extended, a TFTusing a crystalline silicon film cannot be put into practice because ofthe problems of withstand voltage. The large fluctuation in thresholdvoltage is another hindrance in making the TFT practically feasible.

A TFT using a crystalline silicon film cannot be applied to anintegrated circuit for processing image data because of problemsconcerning the fluctuation in threshold voltage and the change incharacteristics with passage of time. Accordingly, a practicallyfeasible integrated circuit based on the TFTs which can be used in placeof conventional ICs cannot be realized.

To overcome the problems concerning TFTs using a thin film of amorphoussilicon or TFTs using a thin film of polycrystalline or microcrystallinesilicon, a method for manufacturing a TFT using a particular region isknown in the art. The method for manufacturing a TFT comprises steps offorming a region which can be regarded as a single crystal in aparticular region of an amorphous silicon thin film, and then forming aTFT utilizing this particular region. By employing the method, a TFTwhich exhibits characteristics well comparable to those of a transistorformed on a single crystal silicon wafer (i.e., a MOS type transistor)can be obtained.

The above technology is disclosed in JP-A-Hei-2-140915 (the term "JP-A-"signifies "Unexamined Published Japanese Patent Application"). In FIG.2A, the method comprises the steps of forming a region 201 provided as aseed crystal, and then applying heat treatment to perform crystal growthfrom the region 201 as the seed crystal in a direction of an arrow 203to finally crystallize a region of amorphous silicon patterned into ashape 202.

However, in FIG. 2A according to a conventional method, crystal growthoccurs from a region 204 simultaneously with the crystal growth that isinitiated from the region 201 in which the amorphous silicon patternedinto the shape 202 is used as the seed crystal. That is, when the methodof FIGS. 2A and 2B is employed, unwanted seeds of crystal growth isformed additionally in the region 204 to allow crystal growth to occurin plural modes. Thus, a polycrystalline state comprising internalcrystal grain boundaries is obtained. In heat treatment, crystal growthcannot be performed within a desired area.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method whichefficiently forms a region equivalent to (corresponding to) a singlecrystal in an amorphous silicon film provided as a starting film on asubstrate having an insulating surface. Another object of the presentinvention is to provide a thin film transistor (TFT) that is free fromthe influence of grain boundaries. Another object of the presentinvention is to provide a TFT having a high withstand voltage and whichis capable of operating a large current. Another object of the presentinvention is to provide a TFT whose characteristics do not undergodegradation or fluctuation with passage of time. Another object of thepresent invention is to provide a TFT whose performance is wellcomparable to that of a single crystal semiconductor.

According to one aspect of the present invention, there is provided amethod comprising the steps of: forming selectively a layer of a metalelement which accelerates (promotes) the crystallization of silicon incontact with the surface of the amorphous silicon film; and forming aregion equivalent to a single crystal by irradiating a laser light tothe amorphous silicon film while moving the laser light in the directionfor increasing the area of the amorphous silicon film, wherein the laserlight is irradiated while heating the amorphous silicon film.

In another aspect of the present invention, there is provided a methodcomprising the steps of: forming selectively a layer of a metal elementwhich accelerates the crystallization of silicon in contact with thesurface of the amorphous silicon film; patterning the amorphous siliconfilm in such a shape that the patterned area gradually increases fromthe region in contact with the layer of the metal element; and forming aregion equivalent to a single crystal by irradiating laser light whilemoving the laser light in the direction for increasing the patternedarea, wherein the laser light is irradiated while heating the amorphoussilicon film. The amorphous silicon film is formed by plasma CVD, lowpressure thermal CVD, etc., on a substrate having an insulating surfacesuch as a glass substrate or a quartz substrate.

The metal element for accelerating (promoting) the crystallization ofsilicon is at least one selected from the group consisting of Fe, Co,Ni, Ru, Rh, Pd, Os, Ir, and Pt.

The metal layer can be formed selectively by forming a layer of themetal element on the surface of the amorphous silicon film and thenpatterning the resulting layer of the metal element. The layer of themetal element (which may be a layer containing the metal element) can beformed most preferably by forming a layer of nickel silicide on thesurface of the amorphous silicon film by a method which comprises thesteps of, coating the surface of the amorphous silicon film with asolution containing the metal element and then performing heattreatment.

In the above constitution, the step of "patterning the amorphous siliconfilm in such a shape that the patterned area gradually increases fromthe region in contact with the layer of the metal element" correspondsto a step of patterning an amorphous silicon film into a shape 102 inFIG. 1A. In FIG. 1A, the area of the shape 102 increases with an angleof θ from the portion to which a layer 101 is formed in contact with themetal element.

In the above constitution, the step of "forming a region equivalent to asingle crystal by irradiating laser light while moving the laser lightin the direction for increasing the area of the amorphous silicon film"corresponds to a step in FIG. 1B. In FIG. 1B, a laser light isirradiated while scanning (moving) in a direction of an arrow tosequentially allow the crystals to grow from the region 101 in adirection of an arrow 103 in FIG. 1A, thereby forming a region 104equivalent to a single crystal. The laser light is, for instance, anexcimer laser.

The region equivalent to a single crystal is a region which is free ofinternal crystal boundaries (line defects and planar defects). That is,the region equivalent to a single crystal is a monodomain region. Sincepoint defects are present in the monodomain regions, the regions containhydrogen or a halogen element for neutralization at a concentration of1×10¹⁷ to 5×10¹⁹ cm⁻³.

The metal element for accelerating the crystallization of silicon isalso present at a concentration of 1×10¹⁴ to 1×10¹⁹ atoms·cm⁻³. Theconcentration is defined as a minimum based on the data obtained by SIMS(secondary ion mass spectroscopy). The detection limit of SIMS atpresent for the metal element is 1×10¹⁶ atoms·cm⁻³. However, theconcentration of the metal element can be approximated from theconcentration of the metal element in the solution used for introducingthe metal element. That is, the concentration beyond the limit ofobserved value by SIMS can be approximately calculated from the relationbetween the concentration of the metal element in the solution and thefinal concentration observed by SIMS for the metal element remaining insilicon film.

The region equivalent to single crystal further contains carbon atomsand nitrogen atoms at a concentration of 1×10¹⁶ to 5×10¹⁸ atoms·cm⁻³ andoxygen atoms at a concentration of 1×10¹⁷ to 5×10¹⁹ atoms·cm⁻³. Theseatoms originate from the starting amorphous silicon film formed by CVD.

According to another aspect of the present invention, there is provideda method comprising the steps of: forming selectively a layer of a metalelement which accelerates the crystallization of silicon in contact withthe surface of the amorphous silicon film; applying heat treatment toallow the crystals to grow in the direction of the plane of the filmfrom the region which is in contact with the metal element; patterningthe region of crystal growth such that the area gradually increases inthe direction of crystal growth; and forming a region equivalent to asingle crystal by irradiating a laser light to the amorphous siliconfilm while moving the laser light in the direction along which thepatterned area increases, wherein the laser light is irradiated whileheating the amorphous silicon film at 400° to 600° C.

In the above constitution, the step of "applying heat treatment to allowthe crystals to grow in the direction of i the plane of the film fromthe region in contact with the metal element" corresponds to aconstitution of FIG. 5B. In FIG. 5B, an amorphous silicon film 501undergoes crystal growth in a direction of the film plane (in adirection parallel to the surface of the substrate on which the film isformed) 503 from a region 502 in which a layer of a metal element as acrystal seed is formed.

In the above constitution, the step of "patterning the region of crystalgrowth such that the area gradually increases in the direction ofcrystal growth" corresponds to a step in FIG. 6A. In FIG. 6A, heattreatment is effected such that a pattern having a shape 505 isobtained, such that the area thereof gradually increases in thedirection of crystal growth shown with an arrow 503.

In the above constitution, the step of "forming a region equivalent to asingle crystal by irradiating a laser light to the amorphous siliconfilm while moving the laser light in the direction along which thepatterned area increases" corresponds to a step in FIG. 6B. In FIG. 6B,the laser light is scanned and irradiated in a direction of graduallyincreasing the patterned area 505.

To make a general classification, there are two methods for theintroduction of the metal element for accelerating the crystallization.

One of the methods comprises the steps of, forming an extremely thinfilm of the metal on the surface of the amorphous silicon film (or onthe surface of the base film formed under the amorphous silicon film) bya physical method such as sputtering or electron beam vapor deposition.In the above methods, the metal element is incorporated into theamorphous silicon film by forming a film of the metal element in contactwith the amorphous silicon film. In this method, it is difficult toprecisely control the concentration of the metal element to beintroduced into the amorphous silicon film. Moreover, in an attempt toprecisely control the quantity of the metal element to be introducedinto the film by forming an extremely thin film about several tens ofangstroms (Å), it becomes difficult to form a film in a complete form.In this case, island-like film portions of metal element are formed onthe surface of the forming surface. That is, a discontinuous layer isformed. This can be overcome by, for example, molecular beam epitaxy(MBE) and the like. However, in practice, MBE is only applicable to alimited area.

In case crystallization is effected after forming the discontinuouslayer, each of the island-like regions which constitute thediscontinuous layer functions as a nucleus to accelerate thecrystallization. By careful observation of the crystalline silicon filmobtained by the crystallization from the island-like regions, amorphouscomponents are found to remain in a great number. This can be observedby using an optical microscope or on an electron micrograph. Otherwise,this can be confirmed through the measurements using Raman spectroscopy.It is also confirmed that the metal components remain as aggregates in acrystalline silicon film. The crystalline silicon film is finally usedas a semiconductor region, However, when the metal components remainpartially as aggregates, these aggregate portions function asrecombination centers for electrons and holes in the semiconductorregions. These recombination centers induce particularly undesirablecharacteristics such as an increase in leak current of the TFT.

In contrast to the physical methods for introducing metal elementsmentioned above, there is a chemical method for introducing a metalelement for accelerating the crystallization of silicon. This methodcomprises the steps of, providing the metal element into a solution, andadding the resulting solution to the surface of the amorphous siliconfilm or to the surface of the base film on which the amorphous siliconfilm is formed by spin coating and the like. Several types of solutioncan be used depending on the metal element to be introduced into theamorphous silicon film. Typically, a metal compound available in theform of a solution can be used. Examples of the metal compounds usablein the solution method are shown below.

(1) In nickel (Ni), the nickel compound is at least one selected fromthe group consisting of nickel bromide, nickel acetate, nickel oxalate,nickel carbonate, nickel chloride, nickel iodide, nickel nitrate, nickelsulfate, nickel formate, nickel oxide, nickel hydroxide, nickel acetylacetonate, nickel 4-cyclohexylacetate, and nickel 2-ethylhexanate.Nickel may be mixed with a non-polar solvent which is at least oneselected from the group consisting of benzene, toluene, xylene, carbontetrachloride, chloroform, ether, trichloroethylene, and Freon.

(2) When iron (Fe) is used as the catalytic element, an iron saltselected from compounds such as ferrous bromide (FeBr₂.6H₂ O), ferricbromide (FeBr₃.6H₂ O), ferric acetate (Fe(C₂ H₃ O₂)₃.xH₂ O), ferrouschloride (FeCl₂.4H₂ O), ferric chloride (FeCl₃.6H₂ O), ferric fluoride(FeF₃.3H₂ O), ferric nitrate (Fe(NO₃)₃.9H₂ O), ferrous phosphate(Fe(PO₄)₂.8H₂ O), and ferric phosphate (FePO₄.2H₂ O) can be used.

(3) In case cobalt (Co) is used as the catalytic element, usefulcompounds thereof include cobalt salts such as cobalt bromide (CoBr.6H₂O), cobalt acetate (Co(C₂ H₃ O₂)₂.4H₂ O), cobalt chloride (CoCl₂.6H₂ O),cobalt fluoride (CoF₂.xH₂ O), and cobalt nitrate (Co(NO₃)₂.6H₂ O).

(4) A compound of ruthenium (Ru) can be used as a catalytic element inthe form of a ruthenium salt, such as ruthenium chloride (RuCl₃.H₂ O).

(5) A rhodium (Rh) compound is also usable as a catalytic element in theform of a rhodium salt, such as rhodium chloride (RhCl₃.3H₂ O).

(6) A palladium (Pd) compound is also useful as a catalytic element inthe form of a palladium salt, such as palladium chloride (PdCl₂.2H₂ O).

(7) In case osmium (Os) is selected as the catalytic element, usefulosmium compounds include osmium salts such as osmium chloride (OsCl₃).

(8) In case iridium (Ir) is selected as the catalytic element, acompound selected from iridium salts such as iridium trichloride(IrCl₃.3H₂ O) and iridium tetrachloride (IrCl₄) can be used.

(9) In case platinum (Pt) is used as the catalytic element, a platinumsalt such as platinic chloride (PtCl₄.5H₂ O) can be used as thecompound.

(10) In case copper (Cu) is used as the catalytic element, a compoundselected from cupric acetate (Cu(CH₃ COO)₂), cupric chloride (CuCl₂.2H₂O), and cupric nitrate (Cu(NO₃)₂.3H₂ O) can be used.

(11) In using gold (Au) as the catalytic element, it is incorporated inthe form of a compound selected from auric trichloride (AuCl₃.xH₂ O) andauric hydrogenchloride (AuHCl₄.4H₂ O).

Each of the above compounds can be sufficiently dispersed in the form ofsingle molecules in a solution. The resulting solution is applieddropwise to the surface on which the catalyst is to be added, and issubjected to spin-coating by rotating at 50 to 500 revolutions perminute (RPM) to spread the solution over the entire surface.

This method using a solution can be considered as a method for forming afilm of an organometal compound containing a metal element on thesurface of a silicon semiconductor. The metal element which acceleratesthe crystallization of silicon is allowed to diffuse into thesemiconductor in the form of atoms through the oxide film. In thismanner, they can be diffused without positively forming crystal nucleus,and thereby provides a uniformly crystallized silicon film. As a result,the metal element can be prevented from being partially concentrated orthe amorphous component can be prevented from remaining in a largequantity.

The silicon semiconductor can be uniformly coated with an organometalcompound, and then ozone treatment (i.e., treatment using ultravioletradiation (UV) in oxygen) may be performed. In such a case, a metaloxide film is formed, and the crystallization proceeds from the metaloxide film. Thus, the organic matter can be favorably oxidized andremoved by volatilization in gaseous carbon dioxide.

In case spin coating of the solution is effected by rotating at a lowspeed only, the metal component that is present in the solution on thesurface tends to be supplied onto the semiconductor film at a quantitymore than is necessary for the solid phase growth. Thus, after rotatingat a low revolution rate, the spin coating is effected by rotating thesubstrate at 1,000 to 10,000 RPM, typically, 2,000 to 5,000 RPM. Theorganometal compound that is present in excess can be spun off byrotating at high rate, so that the metal component can be supplied at anoptimum quantity.

The quantity of the metal component to be introduced into the siliconsemiconductor can be adjusted by controlling the concentration of themetal component in the solution. This method is particularly useful,because the concentration of the metal element to be finally introducedinto the silicon film can be accurately controlled. In the method ofintroducing the metal element using the solution, a continuous layer canbe formed on the surface of the semiconductor (or on the surface of theundercoating thereof) without forming island-like regions of metalparticles for the crystallization. Then, a uniform and dense crystalgrowth can be effected by a crystallization method with heat treatmentor with irradiation of laser light.

In the foregoing, an example of using a solution has been described, buta similar effect as the one above can be obtained by forming the film byCVD using a gaseous metal compound, and particularly, a gaseousorganometal compound. However, this method using CVD is disadvantageousin that it is not as simple as the one using a solution.

The method for forming the layer by sputtering and the like as describedabove can be denoted as a physical method. The method using a solutionin forming a layer containing a metal element for accelerating thecrystallization of amorphous silicon can be considered as a chemicalmethod. The physical method can be considered as a non-uniformanisotropic crystal growth method using metal elements, whereas thechemical method can be considered as a method for uniform (isotropic)crystal growth.

In the method for manufacturing a semiconductor as described above, thelaser light is irradiated in a direction of gradually increasing thearea of a region in which the seeds of crystal growth are formed. Inthis manner, a uniform crystal growth can be effected to form a regionequivalent to a single crystal.

Further, the laser light is irradiated to the amorphous silicon filmwhich is patterned such that the area gradually increases from theregion in which the seeds of crystal growth are formed to accelerate thecrystallization while heating and scanning the laser light in adirection for increasing the area of the amorphous silicon film. In thismanner, a uniform crystal growth can be effected to form a regionequivalent to a single crystal.

Also, by patterning a silicon film obtained by crystal growth in adirection in parallel with the substrate in such a manner that the areathereof gradually increases, and further by irradiating a laser lightwhile heating and scanning it in a direction of gradually increasing thearea of the patterned film, a region equivalent to a single crystal canbe obtained, because the crystal growth is allowed to occur in a singlemode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C show steps for manufacturing a region equivalent tosingle crystal;

FIGS. 2A and 2B show steps for manufacturing crystalline regionaccording to a conventional method;

FIGS. 3A to 3E show steps for manufacturing a region equivalent tosingle crystal;

FIGS. 4A to 4D show steps for manufacturing a thin film transistor;

FIGS. 5A and 5B show the steps of crystal growth of a silicon film;

FIGS. 6A and 6B show another steps for manufacturing a region equivalentto single crystal;

FIGS. 7A and 7B show another steps for manufacturing a region equivalentto single crystal;

FIG. 8 is a schematically diagram showing a crystallization step; and

FIG. 9 shows a state of irradiating a laser light and therebycrystallizing an island-like active layer region obtained by Patterningof an amorphous silicon film.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1

The present example refers to a case of forming a region equivalent to asingle crystal from an amorphous silicon film formed on a glasssubstrate. The steps for the example are shown in FIGS. 1A to 1C and inFIGS. 3A to 3E.

After forming a silicon oxide film 302 at a thickness of 3,000 Å bysputtering or plasma CVD as a base film on a glass substrate 301, anamorphous silicon film 303 is formed at a thickness of 500 Å by plasmaCVD or low pressure thermal CVD (FIG. 3A).

A nickel silicide layer 304 is formed by spin coating with a solution(nickel acetate solution) containing nickel as the metal element whichaccelerates (promotes) the crystallization of silicon and by heattreating the coated structure at 300° to 500° C. (400° C. in this case)for 1 hour (FIG. 3B).

The nickel silicide layer 304 is selectively patterned using afluorine-based enchant (e.g., a buffered hydrofluoric acid) to formisland-like nickel silicide layer 101. The amorphous silicon film 303 ispatterned to obtain a state in FIG. 1A. FIG. 3C shows a cross sectionalview taken along A-A' in FIG. 1A. The angle θ indicated by 100 in FIG.1A is preferably 90 degrees or less. The patterned amorphous siliconfilm 303 results in a shape 102 shown in FIG. 1A. In one end thereof isformed an island-like nickel silicide layer 101.

A KrF excimer laser light is irradiated while moving the laser in thedirection 305. The laser light is shaped into a linear beam whoselongitudinal direction corresponds to a direction vertical to thescanning direction. By irradiating a laser light, crystal growth occursfrom the region in which the nickel silicide layer 101 is formed in adirection of an arrow 103. The point in this method is that the specimenis heated in 400° to 600° C. during irradiation of the laser light. Inthis manner, a region equivalent to a single crystal (a monodomainregion) 104 is obtained as shown in FIG. 3E. FIG. 3E corresponds to across sectional view taken in B-B' shown in FIG. 1B.

EXAMPLE 2

The present example refers to a method for manufacturing a thin filmtransistor (TFT) using a region equivalent to single crystal 104 asshown in FIGS. 3A to 3E. In FIGS. 4A to 4D, the method for producing aTFT according to the example is described. In FIG. 1C and FIG. 4A, theregion equivalent to a single crystal 104 is patterned to form an activelayer 401 for the TFT.

The cross sectional view taken along C-C' in FIG. 1C is given in FIG.4A. In FIG. 4B, a silicon oxide film 402 is formed at a thickness of1,000 Å to provide a gate insulating film which covers the active layer401. Then, a film based on aluminum and containing scandium is formed ata thickness of 7,000 Å by electron beam vapor deposition, and theresulting film is patterned to form a gate electrode 403. After formingthe gate electrode 403, anodic oxidation is effected in an electrolyticsolution using the gate electrode 403 as the anode, to from an oxidelayer 404 (FIG. 4B).

An impurity region is formed by doping an impurity ion (phosphorus ionin this example). Thus, phosphorus ions are implanted into regions 405and 408 using the gate electrode 403 and the surrounding oxide layer 404as masks. The regions 405 and 408 are used as a source region and adrain region, respectively. In this step, a channel forming region 407and an offset gate region 406 are formed in a self-alignment (FIG. 4C).

A silicon oxide film is formed to a thickness of 6,000 Å as aninterlayer insulating film 409 by plasma CVD. After forming contactholes, a source electrode 410 and a drain electrode 411 are formed byusing aluminum. By finally effecting hydrogenation by heat treatment inhydrogen atmosphere at 350° C., a TFT in FIG. 4D is obtained.

EXAMPLE 3

The present example refers to a method of forming a region equivalent toa single crystal (a monodomain region) by heating and allowing crystalgrowth to occur by utilizing a metal element which accelerates thecrystallization of silicon, and further irradiating a laser light to thecrystallized region. The steps for the present example are shown inFIGS. 5A and 5B and in FIGS. 6A and 6B. An amorphous silicon film (notshown in the figure) is formed to a thickness of 500 Å by plasma CVD orlow pressure thermal CVD. Although not shown in the figure, a glasssubstrate having a silicon oxide film formed on the surface thereof isused as the substrate.

A nickel layer or a layer containing nickel is formed by adding thesubstrate to a nickel acetate solution. The resulting substrate ispatterned to form a region 502 (a square region with a length of 3 μm).By thermal treatment at 550° C. for 4 hours, the region 502 iscrystallized to provide the seeds for crystal growth. Since the region502 is a small region, it can be converted by this treatment into aregion equivalent to a single crystal.

An amorphous silicon film 501 is formed to cover the region 502. It isrealized that a layer 502 containing a metal element (nickel in thiscase) which accelerates the crystallization of silicon is in contactwith a part of the amorphous silicon film 501.

By heat treatment in 400° to 600° C. (550° C. in this example) for 4hours, crystal growth proceeds two dimensionally from the region 502 asthe seed of crystal growth. Microscopically, the crystals grow intothose of acicular or columnar shape because the crystal growth occurstwo dimensionally in a direction parallel with the substrate. Thus, acrystalline silicon film 504 is obtained. The crystal growth 503 occursover a length of 100 μm or even longer. Further, since the heatingtemperature necessary for the crystal growth is 600° C. or lower, aninexpensive glass substrate having a low deformation point can be used.

After the step of crystal growth in FIG. 5B, the crystalline siliconfilm 504 is patterned into a pattern 505 shown in FIG. 6A. An enlargedview for the pattern 505 in FIG. 6A is given in FIG. 6B. Crystal growth508 proceeds as a linear laser light is irradiated while scanning it ina direction of an arrow 507. The specimen is heated to 550° C. duringthe scanning of the laser light. The laser light is a KrF excimer laserhaving a line beam in a direction perpendicular to the scanningdirection. The laser light having a line beam is a rectangular beam spothaving several millimeters in width and several tens of centimeters inlength, with the longitudinal direction provided perpendicular to themoving direction of laser light.

The crystal growth 508 does not occur simultaneously in a plurality ofplaces, but occurs sequentially in the direction of the arrow 508, i.e.,in the same direction of crystal growth in parallel with the substrateaccording to the step of FIG. 5B. Accordingly, crystal growth occurs ina single mode. Further, since the crystalline silicon film 505 is formedin such a pattern that the area thereof gradually increases inaccordance with the direction of crystal growth, a uniform crystalgrowth can be implemented to finally obtain a large single domain(crystal grain). The fact that the sides 509 are provided approximatelyalong the direction of crystal growth is an important factor forrealizing a large single domain, because the edges 509 of the patternprovides the starting points for suppressing the crystal growth.Accordingly, the pattern 505 provides relatively easily a regionequivalent to a single crystal.

The method according to the present example comprises steps ofcrystallizing the amorphous silicon film by heating and utilizing thefunction of a metal element which accelerates the crystallization ofsilicon, and patterning the heated and crystallized silicon film in sucha manner that the crystal growth proceeds smoothly in the direction ofcrystallization. Further, laser light is irradiated along the desireddirection of crystal growth while heating. In this manner, the patternedregion is converted into a region equivalent to a single crystal.

EXAMPLE 4

The present example relates to the shape of the patterns of a regionequivalent to a single crystal (monodomain region) 102 and 505 shown inFIG. 1A and FIG. 6B. The patterns of the regions 102 and 505 arecharacterized in that the area thereof gradually increases in thedirection of crystal growth. The particular patterns are provided toprevent crystal growth from occurring from a plurality of regions duringcrystal growth . If crystal growth proceeds from a plurality of regions,the growing crystals collide with each other to form grain boundaries.In other words, a uniform crystal growth can be effected by graduallyextending the crystal growth from a single starting point; that is, thecrystal growth is effected unimodal to form a region equivalent to asingle crystal.

A uniform crystal growth can be effected by initiating crystal growthfrom a starting point, and gradually enlarging the region of crystalgrowth. In FIGS. 1A and 6B, the area of crystal growth graduallyincreases from the starting point of crystal growth for a desireddistance from the starting point, and the area is maintained constant inthe region exceeding the desired distance.

However, a region equivalent to a single crystal can be formed by usingan amorphous silicon film or a crystalline silicon film formed into apattern as illustrated in FIGS. 7A and 7B. When the amorphous siliconfilm is patterned into a shape as illustrated in FIG. 7A, a layer of ametal element such as nickel which accelerates the crystallization ofsilicon or a layer containing the metal element is provided in contactwith the region 704, and a laser light is scanned in the direction 705and irradiated to allow crystals to grow from the region 704 in thedirection of an arrow 700. Preferably, a laser light having a linearbeam spot whose longitudinal direction is perpendicular to the scanningdirection is used.

Further, crystal growth is allowed to proceed from the region 704 in thedirection 700 by heating and utilizing the function of the metalelement, and after patterning the crystallized region into the shape701, laser light is irradiated while moving it in the direction 705(with heating). Also in this manner, the crystals are allowed to growagain to obtain a region 701 equivalent to a single crystal. The crystalgrowth which occurs in the direction 700 by the heat treatment providesacicular or columnar crystals, and does not provide a region equivalentto a single crystal. More briefly, the crystal growth achieved by theheat treatment produces grain boundaries. However, the crystal growtheffected by using the laser light irradiated while moving the laser inthe direction 705 produces a uniform crystal growth (unimodal crystalgrowth) from the starting point 704, and it finally forms a monodomainregion, i.e., a region equivalent to a single crystal. Further, byforming a region 702 after patterning, for instance, a regionconstituting the active layer of a TFT can be obtained.

By using the pattern in FIG. 7B, it is also possible to perform crystalgrowth in a manner similar to the case of FIG. 7A. That is, in FIG. 7B,the amorphous silicon film is patterned into the shape 706, and afterproviding the metal element for accelerating the crystallization ofsilicon in contact with the portion 708, a laser light is irradiatedwhile moving it in the direction 709. In this manner, crystal growth canbe effected uniformly in the direction 710 from the region 708 providedas the starting point, so that a region 706 equivalent to a singlecrystal can be finally obtained. Also, by forming a region 707 afterpatterning, for instance, a region constituting the active layer of aTFT can be obtained.

The angles 703 and 711 are preferably 90 degrees or less. If the angleis greater than 90 degrees, the crystal growth which occurs from theedges of the pattern 701 or 706 becomes prominent as to effect a crystalgrowth in plural modes. Monodomain region cannot be obtained from acrystal growth which occurs in a plurality of modes, because such acrystal growth results in the formation of plural domains.

EXAMPLE 5

The present example relates to a method for accelerating thehydrogenation (desorption of hydrogen) of amorphous silicon film byplasma treatment to the film. In this manner, the crystallization of theamorphous silicon film is accelerated. In the step of FIG. 3A, plasmatreatment using a plasma of hydrogen or helium is performed to theamorphous silicon film. This step utilizes the ECR condition to obtain aplasma of gaseous hydrogen or gaseous helium under a reduced pressure,and the amorphous silicon film is exposed to the resulting hydrogenplasma.

It is important to heat the amorphous silicon film at a temperature nothigher than the crystallization temperature thereof. The crystallizationtemperature of the amorphous silicon film differs depending on themethod of film formation and the film formation conditions, however, ingeneral, it is 600° to 650° C. The lower limit thereof is about 400° C.Accordingly, the heating temperature is preferably 400° to 600° C. It isalso useful to use the deformation temperature of the glass substrate asa standard for determining the upper limit of the heating temperature.That is, the heat treatment is effected at a temperature as high aspossible, but at such not exceeding the upper limit determined from thedeformation temperature of the glass substrate. By using this method, adesired effect can be obtained while suppressing the influence ofdeformation or shrinking of the glass substrate.

In the treatment using hydrogen plasma, hydrogen that is present in theamorphous silicon film combines with the hydrogen ions of the plasma togenerate gaseous hydrogen. Thus, the desorption of hydrogen from thefilm is accelerated. By effecting plasma treatment using helium, thebond between hydrogen and silicon inside the amorphous silicon film canbe cut by helium ions that collide with the bond. Thus, the bonding ofsilicon atoms with each other becomes accelerated to result in a stateof higher ordering degree in atom arrangement. This state can be denotedas a quasi-crystalline state which is extremely liable tocrystallization.

An amorphous silicon film can be crystallized by applying an energy byheating or irradiating a laser light with a plasma-treated state. Sincethe amorphous silicon film after the plasma treatment is extremelyliable to crystallization, the crystallization can be effected with highreproducibility and results in a film with high crystallinity.

EXAMPLE 6

The present example refers to a method which comprises the steps of,forming a portion as a seed of crystal growth on one edge of anamorphous silicon film formed on a glass substrate, and crystallizingthe entire surface of the amorphous silicon film from the portion byscanning and irradiating a laser light.

The crystallization step according to the example is shown schematicallyin FIG. 8. In FIG. 8, a stage 801 for mounting a glass substrate 802thereon is provided freely movable in a direction opposite to that of anarrow 809. That is, by moving the stage 801, laser light is relativelyscanned in a direction of the arrow 809 to irradiate the glass substrate802. A built-in heater inside the stage 801 heats the glass substrate802 mounted on the stage to a desired temperature. The portion 803 isformed by using a nickel element, and provides a seed for crystalgrowth. The method for forming the portion 803 which functions as a seedof crystal growth is described below.

The amorphous silicon film 804 is formed to cover the portion 803 whichfunctions as a seed for crystal growth. In FIG. 8, a linear laser light808 is irradiated to the amorphous silicon film 804 by relativelyscanning the light 808 in a direction of the arrow 809. A linear laserlight 806 irradiated from a laser irradiation source (not shown) isreflected by a mirror 807 to provide the laser light 808 bentapproximately in a direction perpendicular to the stage 801, and isirradiated to the amorphous silicon film 804. By moving the stage 801 ina direction opposite to that of the arrow 809 while irradiating thelaser light 808, the laser light can be relatively scanned in adirection of the arrow 809. The laser light is irradiated while heatingthe glass substrate 802 in 400° to 600° C., for instance, 500° C., byusing a heater provided to the stage 801.

By irradiating a laser light to a structure in FIG. 8, crystallizationproceeds from the portion 805 of the amorphous silicon film provided onthe portion 803 in which seeds for crystal growth are formed. Since thelaser beam is scanned in a direction approximately in parallel with thediagonal of the glass substrate 802, the crystal growth proceeds in adirection along which the area of the amorphous silicon film 804gradually increases. In this manner, the amorphous silicon film 804 canbe entirely converted into a structure which can be regarded as amonodomain region.

The method of forming a portion 803 which provides a seed of crystalgrowth is described below. After forming an amorphous silicon film 804by plasma CVD or low pressure thermal CVD, the amorphous silicon film804 is patterned to provide a pattern (indicated by 803 in the figure)which functions as a nucleus of crystal growth. After maintaining nickelelement in contact with the surface of the pattern by spin coating, heattreatment is applied thereto for the crystallization to form a portion803 which provides the seed for crystal growth. A specimen as shown inFIG. 8 is obtained by forming an amorphous silicon film 804 to cover theportion 803 provided as the seed of crystal growth.

EXAMPLE 7

The present example relates to a method comprising the steps of, formingan active layer constituting a TFT, and providing a region equivalent toa single crystal by allowing crystal growth to occur from the corner ofthe active layer. FIG. 9 shows a crystallized state of an island-likeregion (active layer) obtained by patterning an amorphous silicon filmand irradiating laser light thereto.

A linear laser light 901 is irradiated to an island-like patternedamorphous silicon film 902. A portion 903 provides a seed for crystalgrowth. The portion 903 can be formed by any method described in theforegoing examples. In FIG. 9, the laser light is irradiated byrelatively scanning the light in the direction of an arrow 905 to allowthe crystal growth to occur from the corner portion of the island-likeregion 902. Thus, an active layer for a TFT comprising a regionequivalent to a single crystal is formed. The active layer 904 isconverted into a region equivalent to a single crystal.

In irradiating a laser light, a portion 903 which functions as a seedfor crystal growth is provided previously so that the laser light 901 isscanned approximately in the direction of the diagonal from the portion903. In this manner, the crystal growth can be proceeded from theportion 903 provided as the seed of crystal growth and in the directionof gradually increasing the crystallized area to finally obtain theentire active layer as a region equivalent to a single crystal.Preferably, the portion 903 provided as the seed of crystal growth isremoved by etching upon completion of the crystallization.

In the method of forming a region equivalent to a single crystal byirradiating a laser light to a silicon film, the crystal growth from aplurality of regions which hinders the formation of a region equivalentto a single crystal can be prevented from occurring by designing apattern for forming the region equivalent to a single crystal and byallowing the crystal growth to occur in a pattern which graduallyincreases the crystallized area from a starting point of the crystalgrowth. Thus, a region equivalent to a single crystal can be readilyobtained in this manner. Accordingly, a TFT free from the influence ofgrain boundaries can be implemented by utilizing the monodomain regionsin the constitution of the TFT. Further, a TFT having a high withstandvoltage and also capable of operating a large current can be obtained.The present invention also provides a TFT free of degradation andfluctuation in characteristics. A TFT having characteristics wellcomparable to those of a TFT utilizing a single crystal semiconductorcan be realized.

What is claimed is:
 1. A method for producing a semiconductor devicecomprising the steps of:selectively forming a layer including a metalelement in contact with an amorphous silicon film; performing crystalgrowth from a portion of the amorphous silicon film which is in contactwith the layer including the metal element in a film surface directionby heating; patterning the amorphous silicon film which hascrystal-grown to increase an area in a direction of the crystal growth;and forming a region corresponding to a single crystal by irradiating alaser light while moving the laser light to a direction in which thearea increases, wherein the laser light is irradiated while heating thesilicon film at 450° to 600° C.
 2. The method of claim 1 wherein themetal element includes at least one of Fe, Co, Ni, Ru, Rh, Pd, Os, Ir,Pt.
 3. The method of claim 1 wherein a concentration of the metalelement in the region corresponding to the single crystal is 1×10¹⁶ cm⁻³to 5×10¹⁹ cm⁻³.
 4. The method of claim 1 wherein the regioncorresponding to the single crystal includes hydrogen or halogen elementat a concentration of 1×10¹⁷ cm⁻³ to 5×10¹⁹ cm⁻³.
 5. A method forproducing a semiconductor device comprising the steps of:exposing anamorphous silicon film to plasma; selectively forming a layer includinga metal element in contact with the amorphous silicon film; patterningthe amorphous silicon film to increases an area from a portion of theamorphous silicon film which is in contact with the layer including themetal element; and irradiating a laser light into the amorphous siliconfilm while moving the laser light to a direction which the areaincreases, to form a region corresponding to a single crystal, whereinthe laser light is irradiated while heating the amorphous silicon film.6. A method for producing a semiconductor device comprising the stepsof:exposing an amorphous silicon film to plasma; selectively forming alayer including a metal element in contact with the amorphous siliconfilm; performing crystal growth from a portion of the amorphous siliconfilm which is in contact with the layer including the metal element in afilm surface direction by heating; patterning the amorphous silicon filmwhich has crystal-grown to increase an area in a direction of crystalgrowth; and irradiating a laser light while moving the laser light to adirection in which the area increases, to form a region corresponding toa single crystal, wherein the laser light is irradiated while heatingthe silicon film at 450° to 600° C.
 7. The method of claim 5 wherein themetal element includes at least one of Fe, Co, Ni, Ru, Rh, Pd, Os, Ir,Pt.
 8. The method of claim 6 wherein the metal element includes at leastone of Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt.
 9. The method of claim 5wherein a concentration of the metal element in the region correspondingto the single crystal is 1×10¹⁶ cm⁻³ to 5×10¹⁹ cm⁻³.
 10. The method ofclaim 6 wherein a concentration of the metal element in the regioncorresponding to the single crystal is 1×10¹⁶ cm⁻³ to 5×10¹⁹ cm⁻³. 11.The method of claim 5 wherein the region corresponding to the singlecrystal includes hydrogen or halogen element at a concentration of1×10¹⁷ cm⁻³ to 5×10¹⁹ cm⁻³.
 12. The method of claim 6 wherein the regioncorresponding to the single crystal includes hydrogen or halogen elementat a concentration of 1×10¹⁷ cm⁻³ to 5×10¹⁹ cm⁻³.
 13. A method forproducing a semiconductor device comprising the steps of:forming anamorphous silicon film on a substrate having an insulating film;patterning the amorphous silicon film to form only one corner portion;holding a metal element in contact with the corner portion;crystallizing the corner portion by heating; forming an anotheramorphous silicon film on the corner portion and the insulating surface;and irradiating a laser light from the corner portion to an oppositecorner portion of the another amorphous silicon film.
 14. The method ofclaim 13 further comprising the step of removing the corner portionafter the irradiating step.
 15. The method of claim 13 wherein amonodomain region is formed in the another amorphous silicon film. 16.The method of claim 13 wherein the metal element includes at least oneof Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt.
 17. A method for producing asemiconductor device comprising the steps of:forming a plurality ofamorphous silicon films on a substrate having an insulating film;patterning each of the amorphous silicon films to form only one cornerportion; holding a metal element in contact with each corner portion;crystallizing the corner portions by heating; forming another amorphoussilicon films on the corner portions and the insulating surface; andmoving a laser light to irradiate the laser light from the cornerportions to an opposite corner portion of the another amorphous siliconfilm, thereby to form crystallized silicon films as active layers. 18.The method of claim 17 further comprising the step of removing thecorner portion after the irradiating step.
 19. The method of claim 17wherein a monodomain region is formed in the another amorphous siliconfilm.
 20. The method of claim 17 wherein the metal element includes atleast one of Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt.
 21. A method ofmanufacturing a semiconductor device comprising the steps of:forming asemiconductor film on an insulating surface; providing saidsemiconductor film with a catalyst metal containing material, saidcatalyst metal being capable of promoting crystallization of saidsemiconductor film; irradiating said semiconductor film with light whileheating said semiconductor film, thereby crystallizing saidsemiconductor film, wherein the semiconductor film has substantially nopoint defects and planar defects and contains hydrogen at aconcentration not higher than 5×10¹⁹ atoms/cm³.
 22. A method ofmanufacturing a semiconductor device according to claim 21 wherein saidsemiconductor film comprises silicon.
 23. A method of manufacturing asemiconductor device according to claim 21 wherein said light is a laserlight.
 24. A method of manufacturing a semiconductor device according toclaim 21 wherein said semiconductor film is formed by CVD.
 25. A methodof manufacturing a semiconductor device according to claim 24 whereinsaid semiconductor film contains carbon and nitrogen at a concentrationof 1×10¹⁶ to 5×10¹⁸ atoms/cm³ and oxygen at a concentration of 1×10¹⁷ to5×10¹⁹ atoms/cm³.